Recently, there has been much emphasis on the development of novel fuel cell technologies as portable high energy density power sources for consumer electronics, military applications, medical diagnostic equipment, and mobile communication. These systems must be lightweight, energy efficient, and able to operate for long periods of time without refueling. This interest in miniaturization of power sources has been expanded to microsystems for powering MEMs and related devices, such as “lab-on-a-chip” systems and micro-pumping assemblies. Merging the development of fuel cells with microtechnology has led to the study of micro-fuel cells and their application to micro-devices, as well as to a myriad of portable systems. Additionally, the Department of Defense (DOD) has frequently expressed a need for high-energy, lightweight power sources for the soldier. The power needs of the individual warrior is the main driver behind the DOD search for power sources that are lighter, can deliver more power, have longer running times and have fewer overall logistic problems. Today's soldier is burdened with 16 different batteries weighing 2.5 pounds. With the new Army vision of the Land Warrior, version 1, the total number of batteries should be reduced to 4 and the weight should be reduced to 2.0 pounds. In the future, an Army soldier is expected to have >1 KW of power on a 72-hour mission carrying even less weight. Such goals can only be met by a combination of rechargeable batteries and fuel cells.
Micro fuel cells, which are similar to conventional low temperature fuel cells, rely on a polymer electrolyte membrane (PEM; part of the membrane electrode assembly) as an ionic conductor (for generated protons) and as a barrier for separating the oxidizer and fuel. Either or a variety of simple organic fuels, such as methanol or ethanol, can be used as a fuel. The oxidizer is typically oxygen from air.
One of the most challenging aspects of the miniaturization of fuel cells is the PEM, which suffers from numerous problems including: drying out of the membrane (especially at high operating temperatures), fuel crossover into the oxidizer, in addition to the high expense typically associated with membrane development. All of these problems are further compounded by the need to decrease the thickness (further increasing the complication of the network structure) of the PEM when designing a micro-fuel cell. Incorporation of a PEM has been achieved in a number of micro-fuel cells studied to date. A number of these are biofuel cells. Recently, there have also been a number of biofuel cells that employ enzymes as catalysts at both the anode and cathode surfaces in order to achieve some degree of selectivity to the fuel/oxidizer thus decreasing the problem of fuel crossover and eliminating the need for a PEM. While these enzymatic redox systems can provide the desired selectivity, they typically generate very low power and suffer from all of the problems attendant to the use of enzymes, with long-term stability being especially problematic. The PEM also takes up much of the space in the non-enzymatic micro-fuel cells being developed, thus limiting the size of the final device. Despite the significant advances in PEM fuel cells that have been achieved in the last decade, there are still a number of unresolved issues that have limited their use. The polymer electrolyte membrane is an expensive and often unreliable component of PEM fuel cells. Thus, one of the more serious complicating factors (among numerous others) in the miniaturization of fuel cells has been the instability, under operating conditions, of the membrane electrode assembly, which incorporates the proton exchange membrane.
The dimensions of the fuel cell can be reduced, the time and effort required for fabrication and system integration can be reduce, and cost can possibly be reduced by elimination of the PEM, as well as eliminating the need for enzymatic selectivity, in micro-fuel cell designs.
Currently, laminar flow has been employed in developing micro-fuel cells. It has been demonstrated that laminar flow can be used to create a micro-fuel cell with a diffusive interface as the membrane, thus eliminating the need for a PEM. One design is based on a Y-shaped microchannel injected with two fuels side by side and is described in a paper by Choban et al. Electrodes whose areas were enhanced with nanoparticles of Pt were deposited in a two-step process requiring a complex evaporation step and subsequent electrodeposition step. These electrodes were positioned on one side (the bottom) of the microchannel, separated by a gap, to form the anode and cathode, respectively. However, the complicated evaporation techniques necessary, fabrication limitations, as well as the lack of versatility of this micro-fuel cell platform make this design somewhat impractical. Moreover, as the size of the interface between the two fuels is defined by the depth and length of the channel, the interface area remains the same no matter how wide the channel is made. In one previously reported device, the depth of the deepest channel described was 200 microns, so for each millimeter of channel length, there is only 0.2 mm2 of interface area (0.2 mm depth×1 mm length). The interface area per unit of fluid volume (mm3) can also be calculated. The width of the channel is described as being 2 mm. Therefore, there is 0.4 mm3 (0.2 mm depth×1 mm length×2 mm width) of total fluid per millimeter of channel length and the interface area per cubic millimeter is 0.5 mm2 (0.2 mm2/0.4 mm3). The only way to increase the interface area is by increasing channel depth (which may be difficult or costly to achieve using certain manufacturing techniques such as photolithography [which has difficulty producing large vertical walls] or to do without at the same time increasing channel length) or channel length (the maximum useful length may be limited by the dynamics of parallel and laminar flow.
U.S. Pat. No. 6,713,206, issued on Mar. 30, 2004 to Markoski et al. (hereinafter “'206 patent”) and U.S. Patent Publication No. 20040072047, published Apr. 15, 2004, also to Markoski et al. (hereinafter “'72047 Publication”), teach the use of laminar flow induced dynamic conducting interfaces for use in micro-fluidic electrochemical cells generally, including batteries, fuel cells, and photoelectric cells. In the approach described in the '206 patent, the laminar flow regime is instituted by injecting two fluids in mutually opposed injection geometry. The '206 patent teaches that “[a] T-junction (FIG. 4A) brings two miscible streams together in a laminar flow, which is maintained without turbulent mixing. In contrast, introducing the two streams in an arrow-type junction (FIG. 4B) produces turbulent flow and subsequent mixing.” The '206 patent does not suggest that any angles of relative orientation of the arrow junction are acceptable to produce laminar flows, but that only mutually opposed flows should be used. However, in the '72047 Publication, a laminar flow fuel cell structure is described which incorporates a Y-shaped channel. The structure used in the '206 patent for testing purposes that are reported in the patent involved electrochemical cells in which electrodes measuring approximately 0.125 mm thickness×20 mm length×3 mm width were placed in side-by-side configuration with approximately 5 mm gap between them, as shown in The '206 patent FIG. 7. In the examples described in the '206 patent, using the geometry of FIG. 7, a laminar flow regime appears to have been set up over an area of some centimeters in length by a depth of the thickness of a glass cover slip, which thickness dimension is not reported. Sources of supply for glass cover slips having thicknesses in the range of about 0.1 mm to 0.4 mm are readily located on the Internet. Even assuming a thickness of 0.4 mm, the laminar flow interface that is described in the '206 Patent in the examples provided would be no larger than 0.4 mm=0.04 cm height×20 mm=2 cm length, or approximately 0.08 cm2 in area, which is quite small. The interface area between the two fluids per millimeter of channel length as they flow in contact through the laminar flow channel can be calculated for this device using dimensions given in the '206 patent as 0.4 mm2 (0.4 mm depth×1 mm length) per millimeter of channel length. The interface area per unit volume can also be calculated. Assuming the channel width is at least 11 mm (the bottom of the channel has two electrode strips side by side with a 5 mm gap between them, and the width of the two electrodes is described as 3 mm each), there is 4.4 mm3 of fluid (0.4 depth mm×11 mm width×1 mm length) per millimeter of channel length. Therefore, the interface area per unit volume for the device described in the '206 patent is 0.91 mm2 (0.4 mm2 area×4.4 mm3 volume) per cubic millimeter of fluid. The device shown in FIG. 13 of the '72047 Publication is described as having a 1 mm by 1 mm channel. The interface area in this device per millimeter of channel length is therefore 1 mm2 and the interface area per cubic millimeter of fluid volume is 1 mm2 (volume=1 mm width×1 mm length×1 mm depth). Since the amount of a substance that can be caused to react is proportional to the area, of the interface between the two laminar flows, one problem that needs to be solved is how to arrange for larger areas of the interface between such laminar flows and also to increase the interface area without also increasing the volume of fluid. There have also been studies for the development of sensors taking advantage of laminar flow, as well as using the diffusive interface for microfabrication of small wires or polymer strands inside microchannels. Moreover, much theoretical work has been done in order to understand the mixing behavior of two solutions flowing in a laminar fashion side by side.
Those familiar with fluid dynamics are aware that fluids flowing in a channel without being stirred, flow in a laminar flow regime when the Reynolds number is less than about 2100, and that they flow in a turbulent regime when the Reynolds number is higher than about 2100. The Reynolds number is a dimensionless number given byNRe=ρvL/μ,where ρ (rho) is the density of the fluid, v is the linear velocity, L is a characteristic length, and μ (mu) is the viscosity of the fluid. The Reynolds number is defined as the ratio of inertial forces to viscous forces, and can be computed using a variety of different units of measure, so long as the result is a dimensionless value. Other dimensionless numbers that can be used to characterize flowing fluids are the Schmidt number NSc and the Peclet number NPe.
There is a need for a structure in which a laminar flow interface having appreciable areas can be produced, in which significant amounts of reagents can be controllably reacted.